Shane,
We would like to start monitoring our two smtp servers. They are fairly busy boxes, maybe 100,000 messages a day, give or take several thousand. They of course run Spamassassin, Postfix is also used. We use MRTG to monitor internal servers and switches, and would really like something with a similar graph.
If you are using amavisd to call SpamAssassin (instead of spamd), then there is a huge number of SNMP counters and gauges to monitor the operation in real time, including SNMP gauges on current Postfix queue sizes. See: http://www.ijs.si/software/amavisd/AMAVIS-MIB.txt for a list of available OIDs. It involves a lightweight daemon amavisd-snmp-subagent, which collects statistics from amavisd statistics database in almost- real time, and presents itself as na AgentX to a Net-SNMP daemon, which deals with the SNMP protocol. We are using Cacti for monitoring a selection of these stats. No need to parse log files. See also http://www.amavis.org/amavis-2011.pdf slide 90 and onwards. Mark